wind chill heat index

What are Wind Chill and Heat Index?

Wind chill and heat index are apparent temperatures, or perceived temperatures.  This is how the temperature “feels”.  They are actually calculations that include not only temperature, but also wind and humidity respectively.

Wind Chill

Wind chill, or wind chill factor, is the perceived drop in temperature due to the wind.  The faster the wind is moving, the lower, and farther away from the actual temperature the wind chill will be.

Why?  Well, there is a reason for this.  When things lose heat through convection, that heat is lost to the adjacent air which is cooler, thus warming it up slightly.  When the wind is blowing, it moves that insulating layer of warm area around, speeding up the heat loss.  The faster the wind blows, the more heat loss that occurs.

Here’s a copy of the National Weather Service’s Wind Chill Chart.  If you click on the image, it’ll also take you to their wind chill calculator.

national weather service wind chill chart
So, why does wind chill even matter?  Well, as you can see on the chart above, wind chill helps determine the onset time of frostbite.  Having exposed skin when it’s very cold, cold and windy enough, can be very dangerous.

Heat Index

Similarly to wind chill that factors in wind speed with the temperature, heat index factors in relative humidity with the temperature.  Conversely to wind chill which is always lower than the actual temperature, heat index is always higher.  This is the National Weather Service’s Heat Index Chart.

Who cares about the heat index?  You should!  Prolonged exposure to heat can do lots of damage just like the cold.  Have you heard of Heat Advisories, Excessive Heat Watches, and Excessive Heat Warnings?  They are basically the orange and red areas of the chart above.

Heat Advisory

Advisories are issued when the maximum heat index temperature will exceed 100° for at least two consecutive days.  The criteria does differ across different regions of the country.

Excessive Heat Watch

Heat watches are issued when conditions are favorable for an excessive heat event in the coming 24-72 hours, but the occurance and timing are still uncertain.

Excessive Heat Warning

Heat Warnings are issued when extremely dangerous heat conditions are present.  A rule of thumb is when the maximum heat index temperature will exceed 105° for at least two consecutive days.  Again, the criteria for heat warnings also differ across different regions of the country.

Summary

Ok, so as complicated as wind chill and heat index sound to calculate, the important thing to remember is that an extreme in either can be dangerous.  This is the reason you see them posted on the Weather Channel or during the weather of the local news.   In addition to weather forecasts on television, many home weather stations calculate wind chill and heat index for their users. 

MAP: Current Wind Chill / Heat Index

Children are expecially prone to issues due to extreme “feels-like” temperatures.  Here’s a chart that includes both extremes.  It provides both wind chill factor and heat index values that everyone should be aware of for children.


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